Small-sized variable magnification optical system

Abstract

A variable magnification optical system comprises at least three optical units which are a first moving optical unit, a fixed optical unit and a second moving optical unit. The three optical units are arranged in that order in a propagation direction of light, and a variation of magnification is effected by a relative movement between the first moving optical unit and the second moving optical unit. If a ray which exits from an object and enters the variable magnification optical system, and passes through a center of a stop of the variable magnification optical system and reaches a center of a final image plane is represented as a reference axis ray, the second moving optical unit has a cross-sectional shape which is asymmetrical in a plane which contains the reference axis, and a curved reflecting surface which is inclined with respect to the reference axis, and the direction of the entering reference axis and the direction of the exiting reference axis of the second moving optical unit are parallel to each other and differ from each other by 180°, the variable magnification optical system being arranged in such a manner that a final image is formed after an intermediate image is formed at least twice.

What is claimed is:  
     
       1. A variable magnification optical system comprising a fixed optical unit and a plurality of magnification varying optical units which are arranged in that order in a propagation direction of light, a variation of magnification being effected by a relative movement between said plurality of magnification varying optical units, 
       wherein letting f i  be a focal length of any magnification varying optical unit i and letting k be a number of times by which an on-axial light beam forms an intermediate image in said any magnification varying optical unit i, said any magnification varying optical unit i satisfies:  
       
         
           f i ·(−1) k >0(k is an integer not less than 0),  
         
       
        and wherein if a ray which exits from an object and enters said variable magnification optical system, and passes through a center of a stop of said variable magnification optical system and reaches a center of a final image plane is represented as a reference axis ray; a reference axis ray which is incident on any surface of said variable magnification optical system or enters any of said optical units is represented as an entering reference axis of said any surface or said any optical unit; a reference axis ray which exits from said any surface or said any optical unit is represented as an exiting reference axis of said any surface or said any optical unit; a point at which the entering reference axis intersects with said any surface is represented as a reference point; a direction in which the reference axis ray travels from an object side toward an image plane along the entering reference axis is represented as a direction of the entering reference axis; and a direction in which the reference axis ray travels from the object side toward the image plane along the exiting reference axis is represented as a direction of the exiting reference axis,  
       any of said-magnification varying optical units includes at least one concave reflecting surface the entering and exiting reference axes of which are inclined with respect to a normal to said concave reflecting surface at the reference point thereof, said concave reflecting surface having a cross-sectional shape which is asymmetrical in a plane which contains the entering reference axis and the exiting reference axis.  
     
     
       2. A variable magnification optical system according to claim  1 , wherein the entering reference axis of a surface which is closest to an object side in said any magnification varying optical units and the exiting reference axis of a surface which is closest to an image side in said any magnification varying optical units are parallel to each other, said magnification varying optical units effecting a variation of magnification by moving in parallel with the entering reference axis. 
     
     
       3. A variable magnification optical system according to claim  2 , wherein said concave reflecting surface has a shape which is symmetrical with respect to the plane (Y, Z plane) which contains the entering reference axis and the exiting reference axis. 
     
     
       4. A variable magnification optical system according to claim  3 , wherein an optical path length, which extends from a first surface numbered from the object side of said fixed optical unit to the final image forming plane of said variable magnification optical system, varies during the variation of magnification. 
     
     
       5. A variable magnification optical system according to claim  4 , wherein the direction of the entering reference axis of said surface which is closest to the object side in said any magnification varying optical unit and the direction of the exiting reference axis of said surface which is closest to the image side in said any magnification varying optical unit differ from each other by 180°. 
     
     
       6. A variable magnification optical system according to claim  5 , wherein focusing is effected by a movement of at least one of said plurality of magnification varying optical units. 
     
     
       7. A variable magnification optical system according to claim  5 , wherein said magnification varying optical units are two in number and each of said magnification varying optical units includes at least three concave reflecting surfaces each of which is identical to said concave reflecting surface. 
     
     
       8. A variable magnification optical system according to claim  7 , wherein any of said magnification varying optical units has a negative focal length and forms an on-axial light beam as an intermediate image once. 
     
     
       9. A variable magnification optical system according to claim  8 , wherein any of said magnification varying optical units includes five reflecting surfaces which continuously reflect a ray, said first, third and fifth surfaces numbered from the object side being concave reflecting surfaces each of which is identical to said concave reflecting surface. 
     
     
       10. A variable magnification optical system according to claim  9 , wherein, letting D(i−1) be a distance along the reference axis from an (i−1)st reflecting surface to an i-th reflecting surface on the reference axis and letting Di be a distance along the reference axis from the i-th reflecting surface to an (i+1)st reflecting surface, a distance between each of said reference surfaces of any of said magnification varying optical units satisfies the following condition:        0.8   <     (     Di     D        (     i   -   1     )         )     <     1.2   .                     
     
     
       11. A variable magnification optical system according to claim  9 , wherein said reflecting surfaces of each of said magnification varying optical units are surface mirrors, respectively. 
     
     
       12. A variable magnification optical system according to claim  9 , wherein each of said magnification varying optical units includes an optical element which is formed as a transparent body on which a plurality of internal reflecting surfaces as well as an entrance refracting surface and an exit refracting surface for a light beam are formed. 
     
     
       13. A variable magnification optical system according to claim  7 , wherein, letting d1 be an amount of movement, during the variation of magnification, of a magnification varying optical unit which is closer to the object side between said two optical units, letting d2 be an amount of movement of a magnification varying optical unit which is closer to the image side between said two optical units, and letting L W  be a value of the optical path length from the first surface to the final image forming plane of said variable magnification optical system for a wide-angle end, and letting L T  be a value of the optical path length for a telephoto end, the following condition is satisfied: 
       
         
           L T =L W +2(d2−d1).  
         
       
     
     
       14. A variable magnification optical system according to claim  7 , wherein, regarding any of said concave reflecting surfaces of each of said magnification varying optical units, letting R y  be a radius of curvature of a paraxial region of said any concave reflecting surface in a plane (a Y, Z plane) which contains the entering and exiting reference axes at the reference point of said any concave reflecting surface, letting R x  be a radius of curvature of a paraxial region of said any concave reflecting surface in a plane (an X, Z plane) which contains the reference point and a center of curvature of the radius of curvature R y  and is perpendicular to the Y, Z plane, and letting 2θ be an angle made by the entering reference axis and the exiting reference axis, the following condition is satisfied:        0.4   <     (         R   x       R   y       ·     1       cos   2        θ         )     <     2.5   .                     
     
     
       15. A variable magnification optical system according to claim  4 , wherein, in a partial system which is formed by an i-th concave reflecting surface, an (i+1)st reflecting surface, and an (i+2)nd concave reflecting surface which are numbered from the object side among concave reflecting surfaces which continuously reflect a ray in each of said magnification varying optical units, letting R y, i  and R y,i+2  be radii of curvature of paraxial regions in a plane (a Y, Z plane) which contains the entering and exiting reference axes at the respective reference points of the i-th concave reflecting surface and the (i+2)nd concave reflecting surface, the following condition is satisfied:        0.5   <     (       R     y   ,     i   +   2           R     y   ,   i         )     <     2.0   .                     
     
     
       16. A variable magnification optical system according to claim  7 , wherein letting β W  be a lateral magnification for a wide-angle end from a surface which is closest to the object side in a magnification varying optical unit which is closer to the object side between said two magnification varying optical units, to a surface which is closest to the image side in a magnification varying optical unit which is closer to the image side between said two magnification varying optical units, the following conditions is satisfied: 
       
         
           0.5<|βw|<1.5.  
         
       
     
     
       17. A variable magnification optical system according to claim  7 , wherein a lens which does not move during the variation of magnification and has a negative refractive power is arranged between said two magnification varying optical units. 
     
     
       18. A variable magnification optical system according to claim  4 , wherein said fixed optical unit includes an optical element which is formed as a transparent body on which a plurality of internal reflecting surfaces as well as an entrance refracting surface and an exit refracting surface for a light beam are formed. 
     
     
       19. A variable magnification optical system according to claim  18 , wherein the direction of the entering reference axis of a surface which is closest to the object side in said optical element of said fixed optical unit and the direction of the exiting reference axis of a surface which is closest to the image-plane side in said optical element make an angle of 90°. 
     
     
       20. A variable magnification optical system according to claim  18 , wherein the direction of the entering reference axis of a surface which is closest to the object side in said optical element of said fixed optical unit and the direction of the exiting reference axis of a surface which is closest to the image-plane side in said optical element are the same as each other. 
     
     
       21. A variable magnification optical system according to claim  18 , wherein said stop is located on the object side of said optical element of said fixed optical unit. 
     
     
       22. A variable magnification optical system according to claim  21 , wherein a lens having a negative refractive power is arranged on the object side of said stop. 
     
     
       23. A variable magnification optical system according to claim  18 , wherein a lens having a refractive index different from the refractive index of said optical element is secured to either of the entrance refracting surface and the exit refracting surface of said optical element of said fixed optical unit. 
     
     
       24. A variable magnification optical system according to claim  4 , wherein said fixed optical unit includes a plurality of reflecting surfaces each of which the entering and exiting reference axes are inclined with respect to a normal to a corresponding one of said reflecting surfaces at the reference point thereof, each of said reflecting surfaces being a surface mirror. 
     
     
       25. A variable magnification optical system according to claim  24 , wherein said stop is located on the object side of said plurality of reflecting surfaces of said fixed optical unit. 
     
     
       26. A variable magnification optical system according to claim  24 , wherein said a prism is arranged on the object side of said plurality of reflecting surfaces of said fixed optical unit which has an entrance surface and an exit surface in such a manner that the direction of the entering reference axis of the entrance surface and the direction of the exiting reference axis of the exit surface differ from each other by 90°. 
     
     
       27. A variable magnification optical system according to claim  24 , wherein a lens having a positive refractive power is arranged on the image-plane side of said plurality of reflecting surfaces of said fixed optical unit. 
     
     
       28. An image pickup apparatus comprising said variable magnification optical system according to claim  1 , said image pickup apparatus being arranged to form an image of an object to be photographed, on an image pickup surface of an image pickup medium.